diff --git a/acp_adapter/server.py b/acp_adapter/server.py
index 65241f6978dde..f6e0462ec9d1c 100644
--- a/acp_adapter/server.py
+++ b/acp_adapter/server.py
@@ -1868,7 +1868,7 @@ class HermesACPAgent(acp.Agent):
                 # model emits absolute paths under ~/.hermes/workspace and the
                 # edit silently lands outside the editor's workspace.
                 session_tokens = set_session_vars(
-                    session_key=session_id, cwd=state.cwd,
+                    session_key=session_id, session_id=session_id, cwd=state.cwd,
                 )
             except Exception:
                 session_tokens = None
diff --git a/tests/acp/test_server.py b/tests/acp/test_server.py
index 8ec168b0f9495..55f9b42ef014d 100644
--- a/tests/acp/test_server.py
+++ b/tests/acp/test_server.py
@@ -409,6 +409,43 @@ class TestPrompt:
         assert isinstance(resp, PromptResponse)
         assert resp.stop_reason == "refusal"
 
+    @pytest.mark.asyncio
+    async def test_prompt_binds_session_id_into_subprocess_env(self, agent, mock_manager):
+        """The ACP prompt path must bridge the session id into child subprocesses.
+
+        Regression: ``set_session_vars`` was called with ``session_key`` only,
+        leaving the ``HERMES_SESSION_ID`` ContextVar bound to the explicit ""
+        default. Once the session-context machinery is engaged, that empty value
+        is authoritative — so ``_make_run_env`` handed child subprocesses an
+        empty ``HERMES_SESSION_ID`` instead of the session's own id.
+        """
+        from tools.environments.local import _make_run_env
+
+        resp = await agent.new_session(cwd=".")
+        state = mock_manager.get_session(resp.session_id)
+
+        captured: dict[str, str | None] = {}
+
+        def _run(*args, **kwargs):
+            # Runs inside the session context copy set up by prompt().
+            captured["child"] = _make_run_env({}).get("HERMES_SESSION_ID")
+            return {"final_response": "ok", "messages": []}
+
+        state.agent.run_conversation = _run
+        state.agent.model = "test-model"
+        state.agent.provider = "openrouter"
+
+        mock_conn = MagicMock(spec=acp.Client)
+        mock_conn.session_update = AsyncMock()
+        agent._conn = mock_conn
+
+        await agent.prompt(
+            prompt=[TextContentBlock(type="text", text="hi")],
+            session_id=resp.session_id,
+        )
+
+        assert captured.get("child") == resp.session_id
+